The present invention relates to an exhaust gas sensor and an exhaust gas sensor system in which the sensor is used.
As a sensor for detecting a constituent of an exhaust gas from an automobile or the like, such as hydrocarbon (hereinafter referred to as "HC"), carbon monoxide (hereinafter referred to as "CO"), or nitrogen oxides (hereinafter referred to as "NOx") for example, a resistance type sensor has been known, in which oxide semiconductor such as SnO.sub.2 is used as the detector element so that the content is detected on the basis of a change in the resistance of the oxide semiconductor which has absorbed the constituent to be detected. On the other hand, a CO sensor has been proposed in which Pt porous electrodes are formed on both surfaces of a zirconia element and in which one of the porous electrodes is covered with an oxidation catalyst. The exposure of the sensor to a gas containing CO and oxygen causes oxidation reaction of CO in the electrode that is not covered with the oxidation catalyst, the reaction causes the potential of the electrode to be a mixed potential influenced by the CO concentration; whereas, in the electrode that is covered with the oxidation catalyst, the exposure results in the complete oxidation of CO, so that a potential occurs which depends on the oxygen concentration in the gas. Thus taking the potential difference between the electrodes as an output allows the CO concentration in the gas to be detected on the basis of the potential difference.
The former resistance type sensor is characterized in that the output of the detector element made of oxide semiconductor varies with the concentration of oxygen of the exhaust gas. Accordingly, a problem occurs that, even with the same concentration of a constituent to be detected, the output value of the detector thus fluctuates in response to the concentration of oxygen in exhaust gas. For this reason, as disclosed in, e.g., Japanese Provisional Patent Publication No. 5-180794, the feeding of oxygen into exhaust gas by a pump element employing solid electrolyte has been proposed for increasing the oxygen concentration and for decreasing the relative fluctuation of the concentration of oxygen in the gas in order to improve the detection accuracy. This arrangement, however, has a problem that a large amount of change in the concentration of oxygen in exhaust gas makes insufficient the effect for suppressing the relative fluctuation of the concentration by the introduction of oxygen from the pump element and does not allow a sufficient detection accuracy to be obtained. On the other hand, the latter type of sensor using the mixed potential causes the similar problem because the potential of the electrode that is covered with the oxidation catalyst varies with the concentration of oxygen in the gas.